This invention relates to a method and apparatus for continuously heat treating elongate metallic products, such as steel bars and tubes. More particularly, this invention relates to the annealing of elongate steel products, such as bars and tubes, by passing heated products from a rolling mill through an enclosed furnace or chamber, after arranging the products in dense packs, to provide the shapes with a predetermined time versus temperature thermal history. The shapes are advanced through the chamber along a horizontal path that extends transversely of the longitudinal axes of the shapes to maintain proper control of the end to end temperature of each product.
Steel products, for example, steel bars and tubes from a rolling mill, are cooled from the rolling mill temperature to a final handling temperature according to some time versus temperature pattern, the nature of which will have an effect on the final properties of the products. As is understood in the art, hot rolled iron-based alloy products, such as steel bars and tubes, are normally gradually cooled from the rolling mill temperature, which is typically 1700.degree. F. to 1800.degree. F. for the various grades of steel that are widely used in steel bars and tubes for the construction industry. However, it has been found that beneficial properties for certain applications may be imparted to hot rolled steel bars and tubes by providing them with a prolonged residence time at a temperature of approximately 1050.degree.-1300.degree. F., during which residence time the temperature is closely controlled, for example, a temperature controlled to a predetermined temperature .+-.25.degree. F. within such range, a process which has the effect of annealing the steel products.
As may be clear from the foregoing, the critical temperature for proper annealing of a steel article is well below the temperature at which the article is formed by rolling, casting or the like. Thus, if the article is transferred to the annealing chamber rather quickly after it is formed, it will be at a sufficiently high temperature when it begins the annealing cycle to avoid the need to reheat it to its proper annealing temperature, and in this case there will thereby be effected a significant reduction in energy consumption. To achieve this, however, it is necessary to transfer the formed steel products from the forming station to the annealing chamber after cooling them to a suitable annealing temperature after a time period determined by a time calculation model and at a controlled rate. Annealing of steel products while they still retain significant heat from a forming operation, and therefore without requiring that the products be reheated to any substantial degree, will also serve to reduce the required size of the annealing chamber, and consequently the required cost thereof.
Because of the long soak cycle required for proper annealing of steel products, as explained above, a continuous furnace or other controlled temperature chamber for such purpose, namely a chamber where the products to be annealed are introduced at one end and are removed from an opposed end after being slowly advanced through the chamber, is ordinarily quite long to provide adequate residence time for the products. Thus, such a chamber is quite expensive in terms of its investment costs and occupies a great deal of valuable floor space in a mill or factory where it is to be installed. Further, in certain cases, for example, in an existing mill or factory where an existing annealing chamber is to be replaced, adequate floor space simply may not be available to install a conventional annealing chamber of adequate capacity simply because of the heretofore required spacing between like bars or other elongate products as they progress through the annealing chamber to provide proper temperature uniformity and distribution within the chamber.